iPhone 6 not booting or detecting, motherboard issue?

Hello, I have an iPhone 6 that isn't booting. I cannot get a empty battery icon, or a "connect to iTunes" icon when put in recovery mode.I don't have much experience with hardware troubleshooting but here is the information I have gathered:

The issue is likely not with the battery, this device was sent to an Apple Store for a battery replacement 6 months ago. However it shouldn't be excluded as a possibility.

The issue isn't with the display. I disconnected the entire front of the phone (all 4 ribbon cables that are attached to the display) and then plugged the device into a computer. It wasn't being detected by iTunes. However, I can put the device in recovery mode and have iTunes detect it's an iPhone 6 in recovery mode.

Since iTunes can detect recovery mode, does this exclude the possibility for the lightning port to be damaged?

I have a working iPhone 5 and iPhone 6s laying around. Are there any parts of these phones I could swap with the iPhone 6 to test which component isn't working?

Could the iPhone 6's motherboard be put in an iPhone 6s to recover its data?

Parts are not interchangeable between the 5 series, the 6 series and the 6S series.

The logic board "IS" the phone so if you have an issue with it, putting it in another phone housing will not change anything.

The only way to recover data from an iPhone (other than via a backup) is to get the phone working nominally so that the data can be extracted via software...unless of course you spend a million bucks like the FBI did in San Bernandino.

You did some good troubleshooting by removing the screen and ruling out the battery is a safe assumption at this point.

Your best bet is to have the phone looked at by a reputable micro-solderer. They will diagnose the phone, usually on a no fix/no fee basis, and determine if the phone is fixable or the data recoverable.